1. Field of the Invention
The invention relates to the chemistry of biologically active compounds. More particularly to β-functionalized chlorin derivatives that can be used as photosensitizers for a wide range of light irradiation treatments such as photodynamic therapy of cancer, infections and other diseases.
2. State of the Art
Photodynamic therapy (PDT) is one of the most promising new techniques now being explored for use in a variety of medical applications and particularly is a well-recognized treatment for the destruction of tumors. Photodynamic therapy uses light and a photosensitizer (a dye) to achieve its desired medical effect. A large number of naturally occurring and synthetic dyes have been evaluated as potential photosensitizers for photodynamic therapy. Perhaps the most widely studied class of photosensitizers are tetrapyrrolic macrocyclic compounds. Among them, especially porphyrins and chlorins have been tested for their PDT efficacy. Porphyrins are macrocyclic compounds with bridges of one carbon atom joining pyrroles to form a characteristic tetrapyrrole ring structure. There are many different classes of porphyrin derivatives including those containing dihydro-pyrrole units. Chlorins, as referred to in the present invention, are porphyrin derivatives, in which one double bond of the aromatic system in β-position is absent. As examples of tetrapyrrolic macrocyclic compounds used as photosensitizers, Patent Publication No US 04656186 by Bommer et al. discloses fluorescent mono, di- or polyamide of an aminocarboxylic acid and tetrapyrrole containing at least three carboxyl groups, U.S. Pat. No. 7,022,843B1 by MacAlpine et al. provides β,β′-dihydroxy meso-substituted chlorins as photosensitizers, and U.S. Pat. No. 7,166,719B2 by Pandey et al. discloses tetrapyrrole compounds containing a fluorinated substituent where the compound is a chlorin or a bacteriochlorin for PDT diagnostic and therapeutic application.
There are several properties that an effective photosensitizer should accomplish. Among them, a desirable characteristic in order to efficiently destroy deep target tissues is a strong absorption at long wavelength. Many current photosensitizers are not efficient enough as they have low absorption in the red region of the spectrum. Chlorins have the advantage that they possess an intense absorption in the red and near-infrared region of the electromagnetic spectrum. As light of longer wavelength penetrates deeper into the tissue, it is thus possible to treat e.g. more expanded tumors, if the PDT is employed for tumor therapy. Chlorins possessing potential for PDT can either be derived from natural sources or from total synthesis.
If the chlorins are derived from natural compounds they are usually obtained by derivatizing chlorophylls or bacteriochlorophylls, as for example the photosensitizers derived from chlorophyll a of photosynthetic plants and algae disclosed in U.S. Pat. No. 5,330,741 by Smith. Due to the sensibility of the natural compounds this is often difficult and requires vast resources. So, the synthesis of chlorins by total synthesis is an appealing alternative. Methods to prepare chlorins and bacteriochlorins by total synthesis are known in the art. Generally these compounds are prepared by first synthesizing the porphyrin and then converting the porphyrin system to a chlorin or bacteriochlorin system. This step can e.g. be performed by the reduction with in situ generated di-imine or by cis-dihydroxylation with osmium tetroxide (patent EP 00337601B1; patent application WO 09613504A1, patent application WO 00061584A1; C. Bruckner, D. Dolphin, 2,3-vic-Dihydroxy-meso-tetraphenylchlorins from the Osmium Tetroxide Oxidation of meso-Tetraphenylporphyrin, Tetrahedron Lett. 1995, 36, 3295-3298; C. Bruckner, D. Dolphin, β,β′-Dihydroxylation of meso-Tetraphenylchlorins, Tetrahedron Lett. 1995, 36, 9425-9428; F. Rancan, A. Wiehe, M. Nöbel, M. O, Senge, S. Al Omani, F. Böhm, M. John, B. Röder, Influence of substitutions on asymmetric dihydroxychlorins with regard to intracellular uptake, subcellular localization and photosensitization in Jurkat cells, J. Photochem. Photobiol. B: Biology 2005, 78, 17-28; I. Laville, T. Figueiredo, B. Loock, S. Pigaglio, Ph. Maillard, D. S. Grierson, D. Carrez, A. Croisy, J. Blais, Synthesis, Cellular Internalization and Photodynamic Activity of Glucoconjugated Derivatives of Tri and Tetra(meta-hydroxyphenyl)chlorines, Bioorg. Med. Chem. 2003, 11, 1643-1652).
Another class of chlorins possesses a diketo-group in one of the four pyrrolic subunits. However, these diketo-chlorins are not suitable for application in PDT e.g. due to their very weak absorption in the red region. Some different ways can be found in the art to synthesize these kinds of chlorins. A possible way is the direct oxidation of dihydroxychlorins obtained by dihydroxylation for example with 2,3-dichloro-5,6-dicyano-benzoquinone as oxidizing agent (H. W. Daniell, S. C. Williams, H. A. Jenkins, C. Brückner, Oxidation of meso-tetra-phenyl-2,3-dihydroxychlorin: simplified synthesis of β,β′-dioxochlorins, Tetrahedron Lett. 2003, 44, 4045-4049). An alternative method is the oxidation of 2-hydroxyporphyrins to the corresponding diketo-chlorins. This conversion can be accomplished by several oxidizing agents (R. Beavington, P. A. Rees, P. L. Burn; A study on the oxidation of 2-hydroxyporphyrins to porphyrin-α-diones, J. Chem. Soc., Perkin Trans. 1998, 1, 2847-2851). Interestingly, 2-hydroxyporphyrins show in solution tautomerism depending on the solvent and can exist in the keto or the enol form (M. J. Crossley, M. M. Harding, S. Sternhell, Tautomerism in 2-Hydroxy-5,10,15,20-tetraphenylporphyrin: An Equilibrium between Enol, Keto and Aromatic Hydroxyl Tautomers, J. Org. Chem. 1988, 53, 1132-1137). They can be synthesized either by dehydration of the corresponding dihydroxy-chlorins or by conversion of 2-nitroporphyrins (M. J. Crossley, L. G. King, S. M. Pyke, A new and highly efficient synthesis of hydroxyporphyrins, Tetrahedron 1987, 43, 4569-4577). A mild and selective method for β-nitration of porphyrins using Cu(NO3)2 in a mixture of acetic anhydride and acetic acid allows the nitration and metallation with copper in one step for a variety of porphyrins (A. Giraudeau, H. J. Callot, J. Jordan, I. Ezhar, M. Gross, Substituent effects in the electro reduction of porphyrins and metalloporphyrins, J. Am. Chem. Soc. 1979, 101, 3857-3862; J. P. C. Tomé, A. M. V. M. Pereira, C. M. A. Alonso, M. G. P. M. S, Neves, A. C. Tomé, A. M. S. Silva, J. A. S. Cavaleiro, M. V. Martínez-Diaz, T. Torres, G. M. A. Rahman, J. Ramey, D. M. Guldi, Synthesis and Photophysical Studies of New Porphyrin-Phthalocyanine Dyads with Hindered Rotation, Eur. J. Org. Chem. 2006, 257-267). Some further functionalizations of diketo-chlorins are known in the art, for example oxidative transformations (M. J. Crossley, L. G. King, Novel Heterocyclic Systems from Selective Oxidation at the β-Pyrrolic Position of Porphyrins, Chem. Commun. 1984, 920-922) or the synthesis of annulated heterocyclic systems (M. J. Crossley, P. L. Burn, S. J. Langford, S. M. Pyke, A. G. Stark, A New Method for the Synthesis of Porphyrin-α-diones that is Applicable to the Synthesis of Trans-annular extended Porphyrin Systems, Chem. Commun. 1991, 1567-1568). However, these compounds have no relevance with regard to an application in PDT. Thus, there is a need to have effective photosensitizers capable of efficiently destroying deep target tissues by intensively absorbing light in the red and near-infrared region of the electromagnetic spectrum.